(a) Technical Field
The present invention relates to a method for efficiently preparing hollow carbon fibers having an empty space in the cross section thereof by using a supercritical fluid.
(b) Background Art
As concerns about the exhaustion of petroleum resources, and environmental effects increase worldwide, the requirement for fuel efficiency enhancement of vehicles increased as well. In order to satisfy this requirement, a method for reducing the weight of a vehicle has an interest in the automotive community. Out of this interest, carbon fiber composites have immerged as an efficient way of increase fuel economy. In the carbon fiber composite, a carbon fiber supporting an external load has a higher specific gravity than a resin as a parent material. Thus, if the strength of a carbon fiber is maintained, and the specific gravity of the carbon fiber is reduced, weight reduction can be achieved with great effects.
For weight reduction of carbon fibers, research related to development of hollow carbon fibers having an empty space in the cross section thereof has been conducted. Conventionally, when preparing hollow carbon fibers, a fluid is fed into the center of a spinneret during spinning of precursor fibers so as to prepare hollow fibers, and the fibers are stabilized and carbonized so as to prepare hollow carbon fibers. [U.S. Pat. Nos. 5,338,605, and 4,358,017]
However, this method is inefficient because it requires high energy consumption for using and collecting the fluid (gas, liquid). In the most frequently used acrylonitrile copolymer precursor fibers, the fed fluid increases the solidification speed of an extruded spinning solution. Thus, it is difficult to produce carbon fibers with a high strength, and to produce carbon fibers having a diameter appropriate for use as a structural reinforcement. The carbon fibers produced via this method are in most cases used for heat insulation.
Furthermore, it is difficult to melt-spin the acrylonitrile polymer because the polymer has a melting temperature which is lower than a decomposition temperature. Thus, the acrylonitrile polymer is limitedly applied to the production of fibers for clothes through melt spinning using a plasticizer such as water. However, when the water assisted melt-spun acrylonitrile polymer is used as a carbon fiber precursor to produce a composite, these carbon fibers have a low mechanical strength. Thus, it is inappropriate to use the acrylonitrile polymer plasticized with water as a reinforcement member.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.